Differential effect of the calmodulin inhibitor trifluoperazine in modulating cellular accumulation, retention and cytotoxicity of doxorubicin in progressively doxorubicin-resistant L1210 mouse leukemia cells. Lack of correlation between cellular doxorubicin levels and expression of resistance.

Calmodulin inhibitors are effective in enhancing cytotoxic effects of doxorubicin (DOX) in DOX-resistant cells, possibly by enhancing cellular levels of drug. In the present study, L1210 mouse leukemia cells adapted to grow in vitro, in the presence of 0.025 to 0.25 microgram/ml DOX, and identified as L1210/DOX0.025, L1210/DOX0.05, L1210/DOX0.1, and L1210/DOX0.25 were approximately 5-, 10-, 20-, and 40-fold DOX resistant, respectively, compared to parent-sensitive cells (L1210/S). Using a soft agar colony assay and 3-hr drug exposure, the IC50 concentration of DOX in the progressively DOX-resistant (5- to 40-fold) L1210 cells ranged from 0.25 to 2.0 micrograms/ml and from 0.08 to 0.25 microgram/ml in the absence and presence of a non-cytotoxic concentration of 5 microM trifluoperazine (TFP) respectively. Further, based on the observed in vitro cytotoxic response, the IC50 concentration of DOX in the presence of 5 microM TFP was 2.5-, 4-, 6.7- and 8-fold lower than DOX without 5 microM TFP in the L1210/DOX0.025, L1210/DOX0.05, L1210/DOX0.1, and L1210/DOX0.25 resistant sublines respectively. In contrast, the IC50 of DOX in L1210/S cells was approximately 0.05 microgram/ml with or without 5 microM TFP. Cellular accumulation of DOX was 15-50% lower in the progressively resistant L1210 sublines compared to similarly treated L1210/S cells. However, in the presence of 5 microM TFP, cellular accumulation of DOX in the L1210/DOX0.05 and L1210/DOX0.1 but not L1210/DOX0.25 was comparable to the L1210/S cells. Cellular retention of DOX in the absence or presence of 5 microM TFP was comparable in similarly treated L1210/S, L1210/DOX0.05 and L1210/DOX0.1 cells, and a 2-fold reduction in the retention of DOX in the absence versus the presence of 5 microM TFP was apparent only in L1210/DOX0.25 cells. At the IC50 of DOX in the presence of 5 microM TFP, although cellular accumulation of DOX was concentration dependent over the range of 1-20 microM TFP, enhancement in cytotoxicity of DOX was dose dependent at 1-5 microM TFP but not 5-20 microM TFP. In cells treated for 3 hr at the IC50 concentration of DOX alone or DOX plus 5 microM TFP, cellular accumulation of DOX was 7- to 14-fold and 2.5- to 3.5-fold higher, respectively, in resistant than in sensitive cells. Additionally, following treatment for 3 hr at the IC50 dose of DOX in the absence or presence of 5 microM TFP, drug retention at 3 hr was 4- to 6-fold and 1.5-fold higher, respectively, in the resistant versus sensitive cells.(ABSTRACT TRUNCATED AT 400 WORDS)